Method of interface contact



4 a as-'11,

United States Patent Ofiice 3,078,99l Patented Feb. 26, 1963 3,078,9l METHQD @F INTERFACE CQNTACT Franklin M. Watkins, Flossmoor, lit, and David B.

The present invention relates to corrosion of ferrous metals. More specifically, the present invention relates to a method of reducing the corrosion of ferrous metal surfaces by aqueous ammonium nitrate solutions.

There is a Well recognized corrosion problem in industries concerned with the manufacture, storage, transportation and handling of aqueous ammonia-ammonium nitrate solutions. In the handling of such solutions it is convenient to transport and store them in ferrous containers such as drums, tanks and pipelines. However, in view of the corrosive nature of ammonia-ammonium nitrate solutions against ferrous metals many manufacturers now use storage and transportation facilities con structed of aluminum. Aluminum is used because its oxide film renders the metal inert to attack by the ammonia-ammonium nitrate solutions. The remedy, however, is a costly one. Corrosion inhibitors of one type or another have been suggested and attempted With varying degrees of limited success. Stainless steel has also been employed to combat this corrosion problem but has in general been less desirable than aluminum.

One possible answer to this corrosion problem presented by ammonia-ammonium nitrate solutions lies in the utilization of coating materials so as to exclude the salt solution from the ferrous metal surface. This technique has not been employed in ammonium nitrate solution service to any great extent for even if any pin holes remain in the coating after application, or develop during use of the coating, the salt solutions attack the metal surface at these points. The large amount of corrosion product formed in these areas causes the coating surrounding the point of attack to peel and eventually the entire ferrous metal surface is exposed to the corrosive solution.

Copending application Serial No. 149,192, filed concurrently herewith in the name of .David B. Sheldahl, discloses that the provision of a carbon-ferrous metal galvanic couple will serve to'protect the surfaces of the ferrous metal from corrosion due to contact with aqueous ammonium nitrate solutions by formation of a gamma-Fe O film. The present invention is directed to a particularly efiicacious manner of affording the carbon ferrous metal couple and the anodical protection.

in the present invention it has been found that a satis factory carbon-ferrous metal couple can be provided by forming a relatively thin coating containing minute carbon particles, on the ferrous metal surface contacting ammonium nitrate solutions. These carbon particles are in electrically conductive contact with the ferrous surfaces, preferably sufr'icient of the carbon particles are in direct contact with such surfaces to give the desired galvanic protection. Accordingly, in the thin coating, say of about 0.5 to 20 or more mils, preferably about 1 to 10 mils, thickness; in electrical contact with the ferrous surfaces, there is generally provided, for instance, at least about 5% or at least about 30% by weight of the coating of the carbon particles. Advantageously, the elemental carbon particles are at least about 65% of the coating, especially about 70 to 85%. The amount of carbon particles necessary or desired is dependent upon factors such as their surface area, the extent of their electrical contact with the ferrous surfaces, the rate of contacting the ammonium nitrate solution and the presence of other ingredients, e.g. corrosion inhibitors, etc., in the coating.

Thus, only those particles in such electrical contact and in electrical contact with the ammonium nitrate solution are providing the desired galvanic protection and effectively preventing corrosion. A given particle may not directly contact both the ferrous surface and the nitrate solution but the contacts may be established through intermediate electrically conductive materials such as other carbon particles.

The carbon employed in the coating composition can be carbon in its amorphous or crystalline forms, for example, charcoal or graphite and is present in a pulverized or finely divided dispersible state. Generally the size of the carbon particles will be in a range of about 10 to 200 microns, preferably about 20 to 50 microns, in diameter.

The effectiveness of the compositions of the present invention is due to the action of the carbon in inducing the formation of a gamma-R 0 oxide film at ferrous metal surfaces exposed by holes that are initially present or develop in the coating. This oxide film passivates the exposed area and prevents any further attack by the ammonia-ammonium nitrate solutions. Formation of this passive film is due to the fact that the electrical potential existing between the exposed ferrous metal and the carbon particles contacting the ammonium nitrate solution is sufiicient to create intense corrosion at the limited surface area which produces sufiicient current, ordinarily a current density (working) in the range of at least about 0.1 to about 1.5 or more in ma./cm. of exposed ferrous surface in contact with the solution, to provide passivation. The voltages resulting may be at least about 0.7, usually about 0.7 to .75 volts.

The carbon particles are held in coating position by an organic or inorganic binder such as an organic resinous material, paint or similar coating composition. Also the carbon-containing coating can be overlain by another coating composition if desired, although this is not necessary, and it is when the carbon particles in electrical contact with the ferrous surfaces are also in contact with the ammonium nitrate solution that the corrosion inhibition is being aiforded by the galvanic couple. Contact with the solution may be through carbon particles at the outer surface of the coating or at holes or cracks in the coating which are present due to imperfections in the coating, wearing away of the coating, etc. A carbon particle in direct contact with another carbon particle in the coating can be considered as a single particle.

If the viscosity of the binder component will permit, the carbon may be incorporated directly into the binder component by slow addition and stirring. Preferably a suitable solvent, for instance, a ketone such as methyl ethyl ketone or an aromatic solvent such as toluol, xylol, etc. is added to the binder to reduce its viscosity and facilitate dispersion of the carbon particles. After the carbon particles are added and thoroughly mixed with the binder and after application to a steel surface the solvent is then removed as by evaporation to produce the coating composition. The coating composition is usually directly selfadhering when applied to the ferrous surface.

' The binder component of the composition of the present invention can be any of the materials commonly employed as binders in paints and other coating products which are chemically inert to ammonium nitrate solutions and may constitute the essential balance of the coating, preferably about 15 to 35 Weight percent. Examples of suitable materials are organic resins such as vinyl copolymer resins, epoxy polymers, styrene-butadiene copolymers, vinyl chloride resins, polyurethanes, oil-treated isocyanates, acrylic resins, phenolic resins, etc. Inorganic binders such as the silicates may also be used.

' The coating compositions of the present invention may include other ingredients which are notdeleterious to the passivating activity of the carbon particles, for instance,

ingredients commonly found in paints and other coating products as, for example, metal oxide pigments, oils, plasticizers, resins, etc. In actual practice it may be found advantageous to just incorporate the carbon particles in, for example, a paint composition commercially prepared whose formulation includes a suitable binder for the carbon particles, particularly those formulations noted for their superior abrasion resistant qualities, their strong adhesion to ferrous metal surfaces, and inertness to ammoniacal ammonium nitrate solutions.

Also the coating composition may contain manganese compounds or chromate salts insoluble in ammonium nitrate solutions as disclosed and claimed in copending application Serial No. 149,187, filed concurrently herewith in the names of Paul Shapiro, Lawrence V. Collings, David B. Shcldahl and Franklin M. Watkins, or arsenate or arsenite compounls essentially insoluble in ammonium nitrate' solutions and essentially soluble in nitric acid as disclosed and claimed in copending application Serial No. 149,188, in the names of inventors as above, also filed concurrently herewith. These copending applications are directed' to improvements of the present invention, the former eliminating the possibility of the voltage falling below the passivation requirement of the carbon-containing coating compositions of the present invention and the latter providing a passive film of increased stability and therefore a coating composition of improved corrosion resistance.

The ammonium nitrate solutions may vary considerably in composition. lthough our system protects vessels containing aqueous ammonium nitrate solutions greater need and utility reside in protecting vessels employed to handle ammoniaca-l aqueous ammonium nitrate solutions. Generally representative of such solutions encountered in industry and which give rise to the corrosion problem discussed above are those having approximately about 1 to 80 or more percent ammonium nitrate, usually at least about 40 percent, preferably about 60 to 70 percent; about 5 to 35 percent free ammonia, and the substantial balance being water, for instance, about 5-2S percent water. These percentages are by weight. Especially useful solutions are those containing a ratio of free ammonia to water of at least about 1.5 to l by weight.

If desired, greater assurance of protection to the ferrous metal surface can be obtained by employing in addition to the coating composition of the present invention, a ferrous metal-carbon galvanic couple, that is, employing one or more elemental carbon cathodes in contact with the aqueous ammoniacal solution and the ferrous metal and having a metallic conductive path between the carbon and metal surface. Thus a ferrous metal-carbon cathode galvanic couple is established which generates sufficient current in situ, that is, without the use of an external current source to accomplish passivation of areas of the ferrous metal that for some reason are not passivated by the coating composition of the present invention, and more important, supplies sufficient current to assist the maintenance of passivation acquired from the coating composition of the present invention.

The current density created by a ferrous metal-carbon galvanic couple is dependent on the surface area ratio of the active ferrous metal to carbon in contact with the solution. The active ferrous metal is the non-passivated ferrous surface in cont-act with the solution and thus the rate of addition of the ammonium nitrate solution may affect the extent of corrosion protection obtained. Accrdingly, the surf-ace area ratio of ferrous metal to carbon and/or rate of addition should be selected to produce a current density sufficient to passivate the ferrous surface. For example, the current density can be increased by increasing the area of the carbon or by slowing the rate of filling of the container. When the ratio of surface area of the ferrous metal to carbon is about 1:1 to :1 suflicient current density is generated even to passivate ferrous metal containers to which the ammoniacal solutions are rapidly added or to maintain passivity once passivation of said ferrous metal surface is accomplished. Much greater ferrous metal to carbon surface ratios say even up to about 290:1 and even more can be used when the ammoniacal solution contacts the metal slowly. In any event, the differences in electrode potentials at the initial contact of the solution with the electrodes, i.e. the working voltage, however obtained, should be sufiicient to provide current densities necessary for passivation of the ferrous metal. Ordinarily in a current density in the range of at least about 0.1 to about 1.5 or more malom. is suflicient for passivation and the voltage resulting will generally be about 0.7 to .75 volt.

Although the carbon cathode can be suspended into the aqueous ammonium nitrate solutions it is preferred that it be directly connected to the lower portion of the container, e.g. the bottom particularly in the case of containers of large size, such as storage tanks so that the current density generated by the couple is suiiiciently great to passivate the ferrous metal as the solution contacts it. This construction maintains the carbon electrode in contact with the nitrate solution in the vessel. It has been found that connecting the carbon cathode, preferably a plurality of carbon cathodes to the bottom of a large container such as a tank car, required steel/ carbon surface ratios are assured and passivation is accomplished.

The ammoniacal solutions may contain additives well known to the art as corrosion inhibitors in these solutions. Examples of these inhibitors are trivalent arsenic com pounds, for example, arsenic trioxide; an arsenite such as sodium, potassium or ammonium arsenites and sulfides of trivalent arsenic; compounds which contain divalent sulfur linked to an atom of carbon with the remaining valences of the carbon atom linking the carbon atom to nitrogen as, for instance, carbon disulfide, thiocyanates, thiocarboxylic acids, thioamides, etc., (See U.S. Patent No. 2,220,059 to Herman A. Beekhuis et al.); and organic compounds having an SH and an OH group, for instance, as disclosed in U.S. Patent No. 2,613,131 to Marion D. Barnes et al. In fact the presence of these additives, particularly the arsenites, may enhance the passivation even in amounts far smaller than taught as effective by the prior art. The presence of compounds which provide the ammoniacal solution with copper and carbonate ion, for instance, basic cupric carbonate, have also been found to enhance the passivation. Not only is the presence of these additives of advantage in enhancing passivation but once passivation has been accomplished they act to further insure protection.

A particularly effective inhibitor additive is the combination of trivalent arsenic compound, a soluble copper compound and carbonate ions, as disclosed in application Serial No. 5,637, filed February 1, 1960, in the names of Paul Shapiro, David B. Sheldahl and Lawrence V. Collings, herein incorporated by reference. The soluble copper compound can be, for instance, the inorganic compounds such cupric carbonates, hydroxides, sulfates, nitrates, etc. Of the many carbonate ion-producing compounds, the more particularly suitable are the inorganic compounds, for instance, alkali metal and ammonium carbonates. Preferably, the copper and carbonate components are provided by a single compound such as basic copper carbonate.

The following examples are included to further illustrate the present invention:

EXAMPLE I Steel coupons .1" X 5" x were cleaned by sandblasting and coated by dipping into various formulations. Coatings were made by suspending various amounts of graphite (325 mesh) with and without supplemental additives in a commercial glass clear lacquer. Lacquer thinner was used where necessary to reduce consistency. The lacquer was approximately 50% solvent. Coatings containing various amounts of the graphite with and 6 Without supplemental additives in oil-treated isocyanate was 78%. This composition was tested in accordance as a binder were also tested. with the procedure of Example I in the same ammoniacal After proper curing time (about a week) the coatings salt solution. No corrosion or flaking was evident after were gouged with a sharp file, inducing two scratches in 2 months exposure to the ammoniacal salt solution.

the criss-cross pattern extending the length of the coupon. 5 We claim:

The coupons were then placed in eight-ounce French 1. A process for reducing corrosion of ferrous metal square bottles partially filled with about 100 mm. of surfaces by aqueous ammonium nitrate solutions which aqueous ammoniacal ammonium nitrate solution so as to comprises coating said ferrous metal surfaces with a coatallow about half the coupon to extend above the liquid ing composition consisting essentially of minute carbon line. The steel coupons were connected to a Sheppard particles and a binder chemically inert to aqueous am- Potentiometer for measuring potentials. After about a monium nitrate solutions, and contacting said coating day or two, appearance and single electrode potentials with an aqueous ammonium nitrate solution. indicated that the coupon was passive, some of the 2. The process of claim 1 wherein the solution is an coupons were removed and while still wet with solution ammoniacal ammonium nitrate solution. the untouched side was gouged in the manner described 3. The process of claim 2 wherein said coating is of above and the rescratched coupon was then immediately about 6.5 to 29* mils in thickness. reimmersed in test solution. The aqueous arnmoniacal 4. The process of claim 3 wherein the coating contains ammonium nitrate solution contained 66.8% NH NO at least about 65 weight percent of said minute carbon 16.6% N3 and 16.6% H O. The results are shown in particles. Table I below: 5. A ferrous metal container containing an aqueous Table I Percent Coating No. H Percent binder Observations Carbon Other A-.- Vinyl mast Scratched prior to immersionalter about a day corrosion obslerved, coating began to pee 50-isocyannte Do.

38-isoeyanate. Do.

42.5isoeyanate Scratched prior to immersion,

no corrosion.

-1acquer Do.

15lacquer Scratched prior to immersion-- no corrosion in 3 months. {No corrosion. Removed from 13-1acquer do solution, scratched while wet, returned to solution-no eorrosion for over 3 months. Scratched prior to immersioncorroded. Scratched while wet-n0 corrosion.

The data of Table I demonstrates corrosion protecting ammonium nitrate solution, said container being coated properties of the coating compositions of the present inwith a coating composition consisting essentially of mivention. nute carbon particles and a binder chemically inert to said EXAMPLE II ammonium nitrate solution.

A coating composition was prepared by suspending 540 Container 9 claiin 5 Whemin the solution is an gm. of graphite (325 mesh) in 150 gm. of a vinyl chloammomacal ammonium mtrate solufion' ridewimq acetate copolymen 375 grams of methyl 7. The ferrous container of claim 6 wherein the coating butyl ketone and 375 gm. of toluol were also added to 18 about 1 to mus f h assist in suspending the carbon through the resin binder The contamer of 7 W erem t coafmg and to reduce the consistency. The Vinyl chloridewinfl tains at least about 65 weight percent of said minute caracetate copolymer resin was of the following composition: bon Parades- Percent References Cited in the file of this patent Vinyl Chloride 84-87 UNITED STATES PATENTS VmYl. acetate 12-15 96,936 Mariner et a1 Nov. 16, 1869 Male acld 0-8711 2,077,469 Fazel Apr. 20, 1937 The percentage by weight of the graphite in the final 2,366,486 Bruni et a1 Jan. 2, 1945 composition, i.e., after the solvent has been evaporated 2,605,189 Christian July 29, 1952 

5. A FERROUS METAL CONTAINER CONTAINING AN AQUEOUS AMMONIUM NITRATE SOLUTION, SAID CONTAINER BEING COATED WITH A COATING COMPOSITION CONSISTING ESSENTIALLY OF MINUTE CARBON PARTICLES AND A BINDER CHEMICALLY INERT TO SAID AMMONIUM NITRATE SOLUTION. 